The present invention generally relates to air cycle Environmental Control Systems (ECSs) and, more specifically, to an improved air cycle ECS and improved method of conditioning water vapor bearing air while minimizing energy losses that might otherwise occur during water vapor removal.
ECSs are used to provide a supply of conditioned air to an enclosure, such as an aircraft cabin and cockpit. An air cycle ECS typically operates on a flow of bleed air taken from an intermediate or high pressure stage within a jet engine having multi-compression stages or from an auxiliary power unit (APU) having a compressor specifically designed as a source of compressed air to the aircraft air conditioning system. All air compressed by the engine or APU is initially ambient air which may contain substantial amounts of moisture while operating at low altitudes. One important function of the ECS is to remove most of that moisture that would otherwise condense and be supplied as unwanted liquid droplets in the cool supply of conditioned air to the cabin.
Two main ECS types have been used recently and are considered state of the art for an efficient air cycle ECS. These are the 3 wheel and 4 wheel high pressure water separation (HPWS) systems. The reference to 3 wheels relates to the fact that three rotating aerodynamic impellers (namely, a turbine, a compressor and a fan) are tied to one another by a common drive shaft. The bleed air is usually pre-cooled within a primary heat exchanger where the heat is rejected to ambient air, and then flowed to the compressor. The fan moves the ambient air through the heat exchanger while the aircraft is on the ground and no air speed is available to push the air through such exchanger. Because water is removed before the air flow reaches the turbine, the flow is at a relatively high pressure created by the compressor. Hence, the term "high pressure water separation" is used.
Typically following compression in the 3 wheel HPWS system, an ambient air secondary heat exchanger, a reheater, and a condenser are employed for air cooling and water condensation. The secondary ambient air heat exchanger cools the air back to near ambient temperature. Next, the reheater further cools the air and the condenser then completes the cooling process. In the condenser, a temperature is reached where most of the water content has to condense into a liquid form. At a typical design condition for the system, about 69% of the total water content in the bleed air stream exits the condenser in liquid form. Some prior condensation takes place in the secondary heat exchanger and the reheater (which is usually about 20 to 30 degrees F. higher than in the condenser). But the amount of condensation in the reheater is significantly less than what occurs in the condenser, and represents less than about 30% of the overall condensation achieved by the system. In fact, condensation is only incidental to the primary goal of the reheater which is to bring the temperature and energy level back up at the turbine inlet after it has been reduced to condense the water.
After final condensation in the condenser, the liquid water is removed by a water extractor. Typically, about 85 to 95% of the liquid water is removed, leaving an essentially dehumidified air containing from about 5 to not more than about 42 grains of total water content (vapor and liquid) per pound of dry air. The resulting dehumidified air flows to the reheater where it cools the incoming moisture laden bleed air and, in turn, the dehumidified air is reheated. The reheated dehumidified air, which contains essentially no liquid moisture, is then supplied to the turbine for expansion and cooling. A cool, expanded air from the turbine is then flowed into the condenser where the incoming moisture laden bleed air is cooled. In turn, the cool expanded air absorbs a heat load equal to the heat of condensation removed from the moist bleed air and the sensible cooling load corresponding to the temperature reduction imparted to the bleed air while in heat exchange in the condenser. The air from the turbine, which has now been warmed in the condenser, is then directly supplied to the cabin.
A feature of the current 3 wheel HPWS system is that the reheater is used to recover part of the cooling load necessary for water condensation. Thereby, the turbine can transform the extra energy represented by the combined pressure and temperature of the incoming dehumidified air into mechanical energy (i.e., shaft power), and can deliver air at lower pressure and temperature. As air at higher temperature is input to the turbine, the thermodynamic laws of expansion dictate that a larger temperature drop occurs in the turbine, as well as more turbine power. This extra cooling and mechanical power represent a partial "recovery" of the heat energy added to the flow in the reheater that is not passed on to the cabin.
The recovery is, in fact, only minimal in the 3 wheel HPWS because there is only a partial amount of condensation that occurs in the reheater. The majority of the energy related to the water condensation process is exchanged in the condenser and is not returned to the turbine for recovery. Instead, such non-recovered energy finds its way directly into the cabin in the form of higher supply temperature. At a typical design operating point for the 3 wheel HPWS system, only about 12% of the water condensation energy is recovered in the form of mechanical energy. That means that about 88% of the energy removed from the wet bleed air for the purpose of water removal results in an increase of the air temperature supplied to cool the enclosure.
In a 4 wheel HPWS system, a second turbine (i.e., the fourth wheel) is added downstream of the condenser. With both the 3 and 4 wheel systems, the dehumidified air leaving the condenser contains extra heat exchanged in the condensing process. In the 4 wheel system, instead of adding entirely to the cabin supply temperature, that extra heat provides a higher energy level to the air entering the second turbine, and partial recovery of that energy can take place in the second turbine. At typical turbine pressure ratios and efficiencies, the 4 wheel system is generally more energy efficient than the above 3 wheel system but requires more complex rotating equipment--namely, 4 wheels. The 4 wheel system also requires a larger condenser because temperature from the first turbine is higher relative to the turbine in a 3 wheel system. The temperature difference is primarily due to the fact that the first turbine is limited to expanding only a portion of the available pressure ratio from compressor to cabin so that enough pressure is left over for expansion by the second turbine. The temperature difference is also because the first turbine output is typically kept under freezing levels.
Despite the general energy efficiency advantage of the prior 4 wheel design over the 3 wheel design, the former still has disadvantages in terms of energy losses in the water removal process. A significant energy disadvantage is due to the first turbine being located upstream of the primary water condensation means (i.e., the condenser) and the second turbine being located downstream of the primary water condensation means. Thereby, only a single opportunity exists for energy recovery from the primary water condensation means--namely, through the downstream or second turbine. Another energy disadvantage results from the first turbine--condenser--second turbine arrangement in terms of water vapor removal in the condenser and potential icing. Because of the need to condense rather than freeze water vapor in the condenser, the temperature flow from the first turbine and into the condenser typically needs to remain above freezing. Such a temperature limitation imposes a pressure ratio expansion or temperature drop limitation across the first turbine. With a temperature drop limitation, there is an accompanying energy recovery limitation on the first turbine.
As can been seen, there is a need for an improved 4 wheel ECS and improved method for high pressure water condensation and extraction in a 4 wheel ECS, both of which increase efficiency. Specifically, there is a need to minimize energy losses incurred in the water condensation and removal process of an ECS. In particular, there is a need for an ECS to reduce the amount of water condensation energy that is added in the form of heat to the supply stream to the enclosure. There is a further need for an ECS and method of condensation that provides more cooling capacity for a given sized system, or a smaller sized system for a given load.